index.md (2306B)
1 +++ 2 title = 'Paging' 3 +++ 4 5 # Paging 6 ## Page faults 7 Special kind of exception, IV 14. 8 9 Types: 10 - invalid: access to invalid address (e.g. write to RO page) 11 - major (slower): valid, but page not in memory (e.g. file, swapped pages) 12 - minor (faster): valid, page in memory 13 14 Kernel page fault: 15 - invalid: 16 - on kernel address: oops (panic/recovery) 17 - on user address: checks (copy to/from user) 18 - major: kernel memory is paged out (not on Linux, e.g. in Windows) 19 - minor: lazy allocations (e.g. old vmalloc) 20 21 User page fault: 22 - could be anything 23 24 Page fault information: 25 - cr2 register has faulting virtual address 26 - error code on stack has: 27 - bunch of reserved bits 28 - P: PTE present bit set (so page present, maybe fault caused by some other problem) 29 - W: write access 30 - U: user access (so we were in user eecution) 31 - R: PTE had reserved bits set 32 - I: faulted while trying to fetch instruction 33 34 VMA: 35 - contiguous virtual region with given properties (bit flags) 36 - anonymous: no file 37 - stored in red-black tree (for efficiency) and linked list (for traversing in order) 38 39  40 41 Demand paging: 42 - new pages are not mapped onto physical memory 43 - on first access, process page faults 44 - kernel assigns page frame to process, creates PTE, resumes execution 45 46 VMA operations: 47 - mmap: maps new region in adress space 48 - find fitting hole in memory map, creates and links new VMA (in simplest case) 49 - munmap: unmaps region in address space 50 - unlinks and deletes VMA (in simplest case), removes mapped page frames, removes allocated page tables, flushes TLB 51 - mprotect: changes region's protection bits 52 - update VMA protection (in simplest case), update affected PTEs 53 - madvise: give kernel advice on region, like `MADV_DONTNEED` (unmap page frames), `MADV_WILLNEED` (read ahead page frames) 54 - find and split: when you work on a part of a VMA (might even split one into three parts) 55 - merge: merge any adjacent VMAs with same properties (flags) 56 57 Transparent Huge Pages (THP) 58 - kernel transparently allocates THP when possible 59 - map: allocate compound pages (physically contiguous pages) and map as THPs 60 - collapse: replace some regular pages with THP 61 - split: split THP into number of regular pages 62 - compact: memory compaction to allocate compound pages